Process for cooling high temperature gases



Jan. 23, 1968 J, JAFFE 3,364,982 I PROCESS FOR COOLING HIGHTEMPERATUREGASES Filed Nov. 13. 1964 INVENTOB.

JAM ES JAFFE ATTORNEY United States Patent Ofilice 3,364,982 PatentedJan. 23, 1968 James Jatfe, Summit, NJ., assignor to Allied ChemicalCorporation, New York, N.Y., a corporation of New York Filed Nov. 13,1964, Ser. No. 410,927 9 Claims. (Cl. 165-1) This invention relates toan improved means for cooling high temperature gases, such as theeffluent gases from a sulfuric acid decomposition unit.

Commercial gas cooling apparatuses commonly employ brick-lined gascooling towers. Such cooling towers suffer from the disadvantages ofbeing space, time. and cost consuming in construction, operation andmainten'ance.

Attempts have been made to cool high temperature gases using wateroracid-cooled shell and tube-type heat exchangers, but such attempts havenot generally proved to be satisfactory. Such devices are subject tofrequent tu'be failure by reason of the wide temperature diiferentialcreated between the upper portions of the tubes and upper tube sheet,and the cooled portions of the tubes. The upper portions of the tubesare not cooled effectively due to the release of non-condensable gasesfrom the cooling Water or cooling acid stream and due to the difficultyof completely venting the space immediately under the upper tube sheet.

It is an object of this invention to provide an efiicient means forcooling high temperature gases which does not suffer from thedisadvantages previously encountered by the art.

More specifically, it is an object of the invention to provide a meansfor cooling high temperature gases in a shell and tube-type heatexchanger in such a manner as to avoid thermal-shock and consequent tubefailure.

It is another object of the invention to provide an efiicient means forcooling high temperature gases which is economical in construction,space and operation.

Other objects and advantages of the invention will become apparent fromthe following description when taken in conjunction with the drawing andclaims.

In accordance with the invention, I have found that the above objectscan be achieved by causing the high temperature gases, which are to becooled, to pass downwardly through the tubes of a tubular shell andtube-type heat exchanger, while concurrently circulating a liquid cooing medium as a film in the tubes from a layer of said liquid coolingmedium maintained on the top tube sheet. The liquid cooling mediumexiting the tubes is then preferably recirculated to the liquid coolingmedium layer maintained on the top tube sheet.

The accompanying drawing shows, in schematic form, an embodiment of theapparatus which may be employed according to the present invention. Theshell and tube-type heat exchanger is shown in elevational partialsection.

The apparatus, as shown in the drawing, essentially comprises a shelland tube-type heat exchanger 14, comprising a tubular shell 1 withinwhich is disposed a bank of heat-exchange tubes 2 pressed into, orotherwise secured to upper and lower tube sheets 3 and 4 respectively.The tubes 2 project upwardly and beyond upper tube sheet 3, are open atthe top and are provided with weirs 5 at their upper extremities.Tubular shell 1 is provided with a head 6, which may be cooled bycirculating cooling fluid through inlet 19 and outlet 20. Head 6 isfurther equipped with a gas inlet 7 and an internal cooling liquidmedium inlet 8. Shell 1 is also provided with an external cooling fluidmedium inlet 9 at its lower extremity and a corresponding cooling fluidmedium outlet 10 is provided subadjacent to upper tube sheet 3. Base 11of the tubular shell is provided with a gas outlet 12 for the cooledgases and an internal circulating cooling medium outlet 13. Acidreservoir and pump tank 15 and circulating pump 16 are connected with asuitable conduit 17 which connects outlet 13 with inlet 8.

In accordance with the invention, the high temperature gases to becooled enter the top of the shell and tube-type exchanger 14 through gasinlet 7. The high temperature gases could be any one or a mixture of avariety of organic or inorganic gases at temperatures as high as about3,500 F. and upwards. The eflluent gases of sulfuric acid decompositionunits are typically at about 5002,300 F. and are particularly suitablefor use according to the process of the invention. Other illustrativegases and gas mixtures which may be advantageously cooled according tothe invention include: Mixtures of nitrous oxide, nitrogen, oxygen andwater vapor resulting when ammonia is burned with air at 1,750 F.;mixtures of chlorinated hydrocarbons, hydrochloric acid and chlorineresulting when methane is burned with chlorine and mixtures of sulfur,methane, hydrogen sulfide and carbon bisulfide'resulting when methaneand sulfur are reacted catalytically at 1,200" P. Other high temperaturegases, or mixtures of high temperature gases which may be cooledaccording to the process of the invention will readily occur to thoseskilled in the art.

The gas or mixture of gases to be cooled is caused to flow downwardlythrough the heat exchange tubes 2 in contact with a film of a coolingliquid which falls, preferably continuously, along the inner peripheralsurfaces of tubes 2. The cooling liquid is circulated from a layer 18 ofthe same maintained on the top tube sheet by the weirs 5, located at thetop of each heat exchange tube. The cooling liquid is withdrawn fromoutlet 13 in base 11 of the cooling unit and is recirculated by means ofpump 16 and conduit 17 through inlet 8 to layer 18 at the top of theunit. Pump 16 is provided with means to increase or decreasecirculation, thus controlling flow rate. Reservoir or pump tank 15aifords the advantage of providing temporary storage for the circulatinginternal cooling liquid. The cooled gases are withdrawn through outlet12 from the base 11 of the cooling unit.

For increased etficiency, a second cooling medium is circulated alongthe inner surface of shell 1 and along the outer surfaces of heatexchange tubes 2. This circulation is accomplished through inlet 9 andoutlet 10. The second cooling medium may be liquid, such as water, orgaseous, such as air. In the event that a second cooling medium is notemployed in this manner, it is desirable to pre-cool the internalcirculating cooling liquid medium prior to reentry to the cooling unitvia inlet 8, by some external cooling means. Further efficiency isachieved when head 6 is insulated in some manner. For example, the wallsof head 6 may be lined with brick or may be liquid-, e.g. water-cooledas shown in the drawing.

The cooling capacity of the system is afiected by the nature of thegases to be cooled, the circulating mediums employed, materials ofconstruction of the components of the cooler, size of the components ofthe cooler, flow rate of the gases to be cooled and flow rates of thevarious cooling mediums.

The internal circulating liquid medium employed must, of course, becompatible with the gases to be cooled, as well as being non-corrosiveto the material of construction of the upper tube sheet and the heatexchange tubes. It also should be phase-separable from any organic orinorganic products which may be condensed. In most cases of acidmanufacture, various chlorination processes and many other processes,water may be used as the internal circulating cooling liquid medium. Inmost cases, however, and particularly those of acid manufacture, thereis an equilibrium amount of acid present in the hot gases which will beabsorbed by the water, thus forming weak or dilute acid. In a preferredembodiment, the acid corresponding to that which will be formed in theheat exchange tubes is used initially as the internal circulatingcooling liquid medium. Such a choice will facilitate design of thematerials of constnuction of the cooler apparatus. Other acids, organicor inorganic, meeting the above-described tests of compatibility,non-cor rosiveness and phase-separability may be employed however, eventhough not produced inherently in the heat exchange tubes. Acids, andparticularly inorganic acids are preferred, in view of their generallyhigher boiling points than water. The higher boiling point allows moreflexibility in operation since, in the event of failure of thecirculating pump, for example, there would be less chance of the coolingliquid vaporizing and damaging the top tube sheet of the cooler. Othermaterials, such as chlorinated hydrocarbons, could be employed, providedthey meet the indicated tests. To illustrate the above, in the case ofcooling sulfuric acid combustion gases, the hot efiluent gases willcontain some S and water. The former will form dilute H 80 with water,if used as a circulating medium; the latter will form dilute sulfuricacid if more concentrated sulfuric acid is used as the circulatingmedium. Thus it is expedient to use sulfuric acid as the internalcirculating medium in this case and it is seen that no matter whatstrength of H 80 is initially used, some equilibrium strength of diluteH 80 will eventually be formed. The material of construction of the tubewalls and top tube sheet will depend upon the strength of. the acidswith which it comes in contact. For example, use of H 50, in excess of93% will prohibit the use of impervious carbon and dilute H 80 say belowabout 75% strength, will prevent the use of commercial steel. Sincedilute H SO will eventually be formed, it is expedient to employ diluteH 50 initially and design the materials of construction accordingly.Similarly, when the gas cooler is used to cool the combustion gases ofammonia and air, it is expedient to employ an internal circulatingmedium of dilute nitric acid which would be inherently produced in theheat exchange tubes. Dilute nitric acid requires stainless steelconstruction. Organic cooling fluids would not be suitable for use insuch a process since they would be oxidized by the HNO formed. In thecase of cooling combustion gases of methane with chlorine, an internalcirculating cooling medium of dilute hydrochloric acid may be employed,or a mixture of chlorinated hydrocarbon liquids would be satisfactory.

The term dilute acid is to be understood as referring generally toaqueous acids as opposed to anhydrous acids. The term is a relative oneand the percentage strength of the acid, qualifying it as dilute, willvary depending on the particular acid in question, which in turn willdepend upon what strength is commonly regarded as being concentrated forthat acid. For example, for the purpose of this discussion, anythingless than about 75 H 50 may be considered dilute. HNO will form anazeotrope at about 70% strength. Dilute HNO may be considered as beinganything less than about 50% strength. HCl forms an azeotrope at astrength of about 23%. Dilute HCl may be considered as being anythingless than about 20% strength. It is to be emphasized, however, that theterm dilute is an extremely relative one and that consequently the aboveexemplary values are flexible.

The internal cooling liquid medium is circulated from a layer of thesame maintained one the top tube sheet by the weirs 5, located at thetop of each heat exchange tube. The weirs serve to meter the flow ofcooling medium within the tubes. The notches in the weirs may be avariety of shapes depending'upon choice of design, such as triangular orV-shaped. V-shaped notches are preferred since they permitmore easilyregulated flow and provide a continuous flow of cooling medium down theinner peripheral surfaces of the tubes in a swirling type motion, whichinsures more eflicient contact of the cooling medium with the innerperipheral surfaces of the heat exchange tubes. The medium depth of theinternal cooling medium layer, which is required to be maintained on thetop tube sheet, is that depth needed to afford protection fromimpingement of hot gases. This will ordinarily be about /2". There is nomaximum limitation on depth of this layer except the practical one ofeconomy and convenience. A layer depth of greater than 6" would affordno particular operating advantages. Cooling medium layer depths of about/2" to about 6" are generally employed with a preferable depth of about1" to about 3".

The number of heat exchange tubes in the cooler may vary according todesign anywhere from one to several thousandths. The cooling capacity ofthe unit will generally increase with the number of heat exchange tubesemployed. Accordingly, it is desirable, as a practical matter, to use arelatively large plurality of heat exchange tubes. The maximum number islimited only by design considerations, i.e. the fabrication facilitiesof the manufacturer. With steel heat exchange tubes, for example, it isfeasible to construct a unit containing between about 1,000 and 2,000tubes. Size of the heat exchange tubes is more or less standard andcould range in normal design depending upon the material of constructionfrom about A ID. to about 6" ID. or more. In the event that imperviouscarbon is employed as the construction material, for example, apreferred size for such a tube is about 1%" CD. by ID.

The flow rate of circulating internal cooling medium required isinterrelated to such factors as the cooling capacity required, number ofheat exchange tubes in the cooler and flow rate of the gases to becooled. Generally, the higher the cooling capacity which is required,the higher should be the internal cooling medium flow rate. An increasedflow rate of internal cooling medium will contribute to more efficientheat exchange and consequent higher cooling capacity. As a minimum, aninternal cooling medium flow rate of about 0.3 lb./hr./inch of tubecircumference is required in order to maintain a film of coolant on theinner peripheral surfaces of the tubes. This figure may be increased toabout 300 lbs./hr./inch of tube circumference, or higher if required.Normally, flow rates in the range of about 50 to about 150 lbs./hr./inch of tube circumference would be employed. Optimum flow ratesordinarily lie in the range of about to about lbs./hr./inch of tubecircumference. The optimum flow rate values are determined on apractical basis by a trial and error procedure whichinvolves comparingthe economics of increased pumping costs to obtain the higher flow ratesversus the larger heat transfers accomplished by the greater coolantcirculation. V

The flow rate of the circulating external cooling medi um does not haveas large an effect upon the cooling capacity of the unit as does theflow rate of the circulating internal cooling medium. Enough coolantshould be circulated to dissipate most of the heat picked up by the heatexchange tubes. In those cases where a liquid external cooling medium isemployed, e.g. water, it is generally desirable to maintain a flow ratein the range of about 20 to about 120 gallons/hr./inch of tubecircumference.

Selection of the optimum number, size and wetted area of the heatexchange tubes to be used is based upon such considerations as theamount of heat to be transferred, the temperature limitations of thematerials of construction of the tubes and the economics of tube sizesversus the cost. These factors may be interrelated in a manner such asfollows: In an illustrative case wherein it is desirable to cool a gasstream from 1,000 F. to 250 F. using dilute sulfuric acid as theinternal circulating cooling medium, the total heat Q which is to beremoved from the gas stream amounts to about 1,000,000 B.t.u./hr. Thelog mean temperature difference AT, between the hot gas stream and theacid coolant film, will be about 400 F. There are two overallcoefficients of heat transfer involved, one being from the acid to thecooling medium outside the tube, the other being from the hot gas streamto the circulating acid coolant. Assuming that the acid to the coolingmedium is controlling at approximately the coefiicient of 100B.t.u./hr./sq. ft./ F., hereinafter designated as the symbol U, thewetted area required equals A Q =w UAT 400x100 wherein Q, U and AT areas defined above. The 25 sq. ft. value for the required wetted area isnow, by a trial and error procedure, distributed among variouscombinations of tube size and length and number of tubes. The variouscombinations are priced out and graph plotted in order to arrive at thatcombination with the lowest net cost. The optimum tube size and numberfor the required wetting area would, of course, be that combination withthe lowest net cost.

In an illustrative embodiment of a typical cooling cycle employing theprocess of the invention, a mixture of sulfuric acid decompositioneffluent gases at about 690 F. is circulated, at a flow rate of 1,347.7lbs./min., downwardly through heat exchange tubes 2 of cooler 14, asshown in the drawing. The sulfuric acid decomposition efiiuent gasmixture has the following composition.

=25 sq. ft.

The apparatus employed is of the type shown in the draw ing, the heatexchange portion of which consists of a bank of 600 Karbate (imperviouscarbon or graphite) tubes, each of which tubes is 12' in length and 1%"CD. by /3 ID. The internal circulating cooling medium employed is about5% sulfuric acid which is maintained at a depth of about 3" on the toptube sheet 3 by weirs 5 and is circulated downwardly as a falling filmalong the inner peripheral surfaces of the tubes at a rate of about 600gallons/min. Cooling water at 85 F. is circulated at a rate of about2,400 gallons per minute, between the inner surfaces of shell 1 and theouter surfaces of the tubes 2, through inlet 9 and exits through outlet10 at about 105 F. The cooling Water also contacts the undersurface oftop tube sheet 3. The cooled gas mixture is withdrawn through gas outlet12 at a temperature of about 120 F. The sulfuric acid internal coolingmedium is withdrawn through outlet 13 at about the same temperature,viz. 120 F. and is recirculated to the acid layer on top tube sheet 3.

In the above illustration, if the sulfuric acid efiluents are at ahigher temperature when passed through the heat exchange tubes, a largernumber of heat exchange tubes would be required in order to maintain thesame throughput of gases and recover the same at the 120 F. level. Forexample, if the sulfuric acid efiluents are at about 1,800 F., about1,200 heat exchange tubes of the same size would be needed. Two units of600 tubes each, in parallel, could be used advantageously if desired.

An important design consideration in choosing the materials ofconstruction for any shell and tube-type heat exchanger is the uppertemperature limitation which the top tube sheet and tube walls will haveto endure. The upper temperature limitation of a tube sheet is normallydetermined by the pressure differential across the tube sheet and thetemperature differential between the shell and the tube fluids at thetube sheet. One of the advantages afforded by the process of theinvention is that the top tube sheet is kept at temperatures below aboutF., resulting in very low temperature differentials at the tube sheet,which circumstance permits the use of cheaper materials of constructionfor the top tube sheet than would be needed if the same were exposed tovery high temperature differentials. For example, with low temperaturedifferentials, ordinary steel and impervious carbon may be substitutedfor the costly high temperature resistent metals, such as the Hastelloyalloys, for use under other favorable conditions. Similarly, since byadjusting the flow rate of the circulating internal cooling medium thetube wall temperature can be kept below about 200 F.; cheaper materialsmay also be used in the tube wall construction than would otherwise 'bepossible.

A further important advantage of the process of the invention is thatthe layer of internal circulating cooling medium, which is maintained onthe top tube sheet, surrounds the projecting uppermost extremities ofthe heat exchange tubes and thus serves to prevent thermal-shock to theheat exchange tubes.

Additionally, the cooling medium, being circulated internally of thetubes, keeps the tubes clean and minimizes fouling of the tubes bycarbon dust and various other hydrocarbon materials. This feature makesthe invention process admirably suited to the cooling of dirty gasmixtures.

Although the invention has been described with some particularity andsince others skilled in the art will readily be able to makemodifications and innovations over the embodiments described; it should'be understood that I do not wish to be limited except by the scope ofthe appended claims.

I claim:

1. A method for cooling high temperature gases which comprises passingthe gases downwardly through the tubes of a tube sheet-type heatexchanger comprising one or more heat exchange tubes and a top tubesheet, maintaining a layer of a first cooling medium on the top tubesheet, circulating the first cooling medium from said layer as a fallingfilm along the inner peripheral surfaces of the heat exchange tubes, inconcurrent flow with the gases and subjecting the outer peripheralsurfaces of the heat exchange tubes to heat exchange with a secondcooling medium.

2. The method of claim 1 wherein the second cooling medium is water.

3. The method for cooling gases at temperatures up to about 3,500 P.which comprises passing the gases downwardly through the tubes of a tubesheet-type heat exchanger comprising one or more heat exchange tubes anda top tube sheet, keeping the top tube sheet at a temperature belowabout 150 F. by maintaining thereon a layer of a first cooling medium,circulating the cooling liquid medium from said layer as a continuouslyfalling film along the inner peripheral surfaces of the heat exchangetubes in concurrent flow relationship with the gases and cooling theouter peripheral surfaces of the heat exchange tubes with a secondcooling medium.

4. The method of claim 3 wherein a plurality of heat exchange tubes areemployed.

5. The method of claim 4 wherein a dilute inorganic acid is used as thefirst cooling medium.

6. The method of claim 5 wherein the dilute inorganic acid is dilutesulfuric acid.

7. The method of claim 5 wherein the medium is water.

8. The method for cooling the efiluent gases from a sulfuric aciddecomposition unit which comprises passing the gases downwardly througha plurality of heat exchange tubes of a tube sheet-type heat exchanger,including a top second cooling tube sheet, keeping the top tube sheet ata temperature below about 150 F. by maintaining thereon a firstcirculating cooling medium consisting of a layer of dilute sulfuricacid, circulating the dilute sulfuric acid from said layer as acontinuously falling film along the inner peripheral surf-aces of theheat exchange tubes concurrently and in direct heat exchangerelationship with the et'fiuent gases and cooling the outer peripheralsurfaces of the heat exchange tubes with a second circulating coolingmedium.

9. The method of claim 8 wherein the second circulating cooling mediumis Water.

8. References Cited UNITED STATES PATENTS EDWARD J. MICHAEL, PrimaryExaminer.

0 ROBERT A. OLEARY, CHARLES SUKALO,

Examiners

1. A METHOD FOR COOLING HIGH TEMPERATURE GASES WHICH COMPRISES PASSINGTHE GASES DOWNWARDLY THROUGH THE TUBES OF A TUBE SHEET-TYPE HEATEXCHANGER COMPRISING ONE OR MORE HEAT EXCHANGE TUBES AND A TOP TUBESHEET, MAINTAINING A LAYER OF A FIRST COOLING MEDIUM ON THE TOP TUBESHEET, CIRCULATING THE FIRST COOLING MEDIUM FROM SAID LAYER AS A FALLINGFILM ALONG THE INNER PERIPHERAL SURFACES OF THE HEAT EXCHANGE TUBES, INCONCURRENT FLOW WITH THE GASES AND SUBJECTING THE OUTER PERIPHERALSURFACES OF THE HEAT EXCHANGE TUBES TO HEAT EXCHANGE WITH A SECONDCOOLING MEDIUM.